This invention relates generally to driver circuits for liquid crystal displays (LCD's) and particularly to a circuit for compensating for variations in the characteristics of the crystals of such displays.
The use of liquid crystal displays (LDC's) in instances where a visual display is required, is increasing rapidly. Because of their low power consumption and economy, LCDs are being used to replace cathode ray tubes for many uses requiring a visual display. Unlike CRTs, liquid crystals show a rapid change in light output over a narrow range of driving voltages just above a threshold voltage. However, keeping a good grey scale, in a monochrome display, or color uniformity, in a color display, requires accurate control of the LCD drive voltages in the region of the threshold voltage. Manufacturing liquid crystals with uniform characteristics is an achievable goal with existing technology. However, the turn-on voltage characteristics of liquid crystals vary substantially over time because of such factors as temperature changes, aging, thickness of the crystals, and for other reasons. Because of these variations in characteristics, even when the crystals within a given display are manufactured with substantially identical turn-on characteristics, matching the drive voltage circuitry to the turn on voltage of the crystals is extremely difficult. The difficulty is increased when the changes are not permanent. For example, temperature changes can cause substantial changes in the crystal characteristics. However, the temperature changes are not permanent. Accordingly, the light output of a portable LCD can vary substantially as the display is moved from one environment to another. For these reasons there is a need for a circuit for compensating for crystal variations in a LCD. The present invention fulfills this need.